In healthy adults the total daily production of IgA exceeds that of other immunoglobulin classes. Molecules of representative secretory and serum IgA differ both in their predominant molecular forms (proportion of IgA1 and IgA2 subclasses as well as polymers and monomers) and sites of production. The objective of the proposed investigation is to determine molecular-cellular mechanisms that regulate synthesis and catabolism of IgA. A team of investigators experienced in hybridoma technology and molecular cloning will study the biochemical and genetic events that occur at the earliest stages of human B cell differentiation with the particular emphasis on the importance of J chain expression. This low molecular weight glycoprotein, previously thought to be involved in polymerization and transport of immunoglobulins into external secretions, is likely to have a more fundamental role in B cell ontogeny. It may be expressed before or independently of immunoglobulin chains. Therefore, we shall determine the tissue distribution and properties of J chain-positive cells in unstimulated cells and transformed cell lines of human fetal lymphocytes. The quantity, molecular form, and subcellular localization of J chain will be determined by radioimmunoassay and immunohistochemical procedures. The participation of J chain in the process of intracellular assembly will be explored in vitro and in vivo taking advantage of already available cell lines that express only J chain. These cells will be fused with monomeric IgA-secreting cell lines. Native J chain (obtained from cell lysates or as a cloned products) will be examined for its ability to polymerize monomeric immunoglobulin in vitro. The synthesis of IgA by plasma cells and their precursors is regulated by T cells and/or their products. Thus, we shall study the regulatory role of T cells isolated from human colostrum, the quintessential product of the secretory IgA system. T cells from other sites will be also evaluated for their potential to regulate IgA synthesis in vitro. In contrast to our knowledge of the synthetic sites and large quantitites of IgA produced, its catabolism remains undefined. Accordingly, we shall examine the fate and tissue distribution of catabolized IgA.